The present invention relates to a tensioning device including a plurality of tensioning rollers disposed to receive and align tubular workpieces, and at least one sensor which is disposed to scan the edge of the opening of the workpiece; at least one tensioning roller having alignment members which are movable transverse to their direction of rotation to automatically align the edge of the opening with respect to a sewing point.
A tensioning device for sewing machines having at least two drivable tensioning parts to receive and align tubular workpieces is described in Federal Republic of Germany Pat. No. 3,117,198, the disclosures of which are expressly incorporated by reference herein. Two electric motors arranged coaxially in the tensioning part are provided in the embodiment disclosed therein, for driving the tensioning part and for further driving the alignment members. In this known embodiment the alignment members are formed of a plurality of gear wheels which are mounted in slots in a support part belonging to the tensioning part, so that they extend in planes that are radial to the axis of rotation of the tensioning part. In this axis of rotation there is provided a central worm gear with which the said gear wheels engage. The electric motor provided for the driving of the alignment members drives the worm gear and thus the gear wheels. The latter are so arranged in the support part that the outwardly directed circumferential portions of the outside diameter of each gear wheel extend beyond the outside diameter of the tensioning part. The tubular workpiece is pulled over the tensioning parts belonging to the tensioning device so that the circumferential portions of the aforementioned gear wheels which extend beyond the tensioning part grip the workpiece on its inner side, namely in the vicinity of the edge of the opening. The tensioning part which is driven by the corresponding electric motor causes the tubular workpiece which has been pulled on to rotate. In this connection its opening edge is scanned by a sensor, for instance a light barrier. Deviations from a desired position are transmitted to a control by which the electric motor intended for the drive of the alignment members--an adjustable-speed DC motor rotating in one direction of rotation as well as, after reversal of polarity, in the other--is controlled and the worm gear thus travels in the clockwise and counterclockwise directions respectively. The known tensioning device therefore permits a movement of a tubular workpiece which is aligned in accordance with the scanned edge of the opening. Since the alignment members do not permit an increase in the diameter of the corresponding tensioning part, the known tensioning device can be used only for light, elastic sewing material, such as tricot fabrics.
For independently controlled alignment of the edge of the opening of a tubular workpiece consisting of medium-heavy, heavy, and less elastic sewing material, for instance the closed waistband of jeans or the like, the known tensioning device is unsuitable for the following reasons:
1. It is impossible to increase the diameter of the tensioning parts, and because of the nonoptimally developed points of engagement of the alignment members which act on the workpiece during the alignment.
2. The electric motor provided for the drive of the alignment members, because of its relatively small size predetermined by the outside diameter of the corresponding tensioning part, is not able to exert, via the alignment members, such large forces on the tubular workpiece, which has been tensioned with considerable initial tension, as are necessary in order to assure dependable alignment, effected transverse to the direction of rotation, of the scanned edge of the opening, particularly under the condition that the tubular workpiece is not transported by the sewing machine but solely by the tensioning device in accordance with the invention.